Optical communication links employing fiber optics are presently being used with increasing frequency in lieu of conventional electrically conductive lines. This is due to the superior bandwidth of optical links which permits them to carry many communication channels on any given optical link. The fiber optics employed in the optical communication links are transparent fibers which are usually made of glass or plastic. These transparent fibers tend to be relatively thin and fragile and thus are more difficult to work with than conventional electrical wires. Since connectors by their very nature are subject to manipulation by humans, the fiber optic cable must be adequately and properly coupled to the body of the connector so as to protect the transparent fiber therein from damage. Additionally, when optical fibers are coupled together, they must be maintained in both axial and angular alignment with each other to rather close tolerances if light losses in the point of connection are to be maintained within acceptable limits.
Connectors satisfying the aforementioned objectives would be considerably less difficult to construct if the aforementioned problems were encountered only by trained physicists, for example, working in a sophisticated laboratory environment. However, optical fibers are likely to form the basis of large scale communication networks in the future and accordingly they must be connected and disconnected relatively often by ordinary personnel working in the field, without the benefit of sophisticated laboratory equipment, and be subject to environmental conditions found in the field as opposed to those found in the laboratory.
The large number of such connectors which will be required dictates that the connectors employed should be relatively inexpensive, should stabilize the cable in a predetermined position in the connector, prevent shifting the position of the transparent fiber within the connector body and protect the transparent fiber from damage when the cable is twisted or pulled by field personnel.
A fiber optic cable is shown in cross section in FIG. 1 of the drawing. The cable includes an optic fiber 1, an insulating sleeve 2, which, for example, may be manufactured from nylon, a reinforcing fiber sleeve 3 which is employed to resist stretching the fiber optic cable when it is pulled and an outer shell 4, which, for example, may be made of vinyl, to protect the cable and resist undue twisting and bending thereof.
FIG. 2 is a partially cut away section view of a conventional prior art connector. The connector shown in FIG. 2 is a male connector. However, it will be apparent to those skilled in the art that the details with respect to the manner in which the fiber optic cable is attached to the connector would differ little for a female connector. The connector includes an annular body 5 having an annular insertion member 8 disposed at one end thereof. The insertion member includes a sharp edge 7 to facilitate the insertion of insertion member 8 into an exposed end of the fiber optic cable. Body 5 includes an opening 6 for receiving the optic fiber 1 of the cable and for communicating it with the optic fiber in the associated female connector (not shown). A coupling nut 10 is rotatably mounted on body 5 to facilitate the coupling of the male connector to the female connector.
As can be seen from FIG. 2, outer shell 4 of the fiber cable is partially removed from the end of the cable exposing a short length of reinforcing fiber sleeve 3. The insertion member generally urges the exposed portion of the fiber sleeve 3 and its underlying insulation sleeve 2 radially outwardly. To prevent a decrease in the holding ability of the connector as a result of the weaves of the reinforcing fiber sleeve 3 unwinding, the exposed fiber is preferably treated with, for example, epoxy resin. Subsequently, a fastening ferrule 9 made of a pliable metal is placed over the cable, positioned over the exposed portion of reinforcing fiber 3 as well as the adjacent end portion of outer shell 4 and then the ferrule is fastened mechanically with pressure to stabilize the cable.
This fiber optic cable connector in use suffers from the disadvantage that the outer shell 4 tends to withdraw from ferrule 9, as indicated by the outer shell 4' shown in dashed lines in FIG. 2, thereby exposing the internal construction of the cable to the environment. Moreover, since one of the functions of the outer shell is to protect the cable against twisting forces, this protection is lost at a highly critical point in the fiber optic system, namely directly adjacent to the connector itself which is subject to human manipulation. Thus, when the connector of FIG. 2 is used, the optic fiber may be subjected to damage. This is particularly true when less pliable glass fibers are used as the transparent fiber of the optical cable.
Several techniques have been proposed in the prior art to overcome the withdrawal of the outer shell 4 from ferrule 9. For example, it has been proposed to adhere outer shell 4 to ferrule 9 by using a glue, such as epoxy resin, between ferrule 9 and outer shell 4. Additionally, it has been proposed that the adherence between the outer shell 4 and the reinforcing fiber system 3 should be improved. These methods have been found to be not altogether practical. The vinyl outer shell 4 becomes soft with rising environmental temperature which decreases its adherence to the fiber sleeve 3. Using epoxy resin to adhere ferrule 9 to the outer shell 4 is not only a cumbersome operation but it has been found that it is very difficult to achieve sufficient adherence between these two members.